Hearing loss degrades formant coding by weakening synchrony capture across auditory-nerve fibres

Evidence

In the healthy ear, nonlinear AN response properties result in systematic patterns of neural fluctuations synchronized to F0 or higher harmonics near AN characteristic frequencies. In particular, synchrony capture results in the dominance of a single harmonic near a spectral peak on the fine-structure of AN responses to voiced sounds (Delgutte and Kiang 1984 ; Deng and Geisler 1987 ). After trauma-induced SNHL, synchrony capture is reduced, and AN fibres respond to multiple harmonics (Miller et al. 1997 ). These studies focused how synchrony capture effects fine-structure coding; however, this mechanism also sets up a pattern of low-frequency fluctuations across the AN population. In the healthy ear, the fibres that are “captured” by a single harmonic and have responses with relatively flat envelopes. In contrast, fibres tuned to frequencies between formants respond to multiple harmonics and strong rate fluctuations at F0. The amplitude of F0-related neural fluctuations provides a robust code for the formant frequencies. In the ear with SNHL, decreased synchrony capture results in AN fibres that respond to multiple harmonics, thus F0-related fluctuations prevail across all driven frequency channels. The profile of F0-related fluctuations that codes the formants, and that ultimately drives modulation-sensitive midbrain neurons, is diminished by SNHL. — Dijk, Pim van, Physiology, Psychoacoustics and…, 5476

Categorization

• XREF: Directly relevant to the reader’s purpose of understanding auditory coding mechanisms; connects to psychoacoustic work on envelope-F0 coding and to clinical audiometry puzzles of discrepancy between audiogram and speech-in-noise performance.
• QUESTION: The mechanism moves coding of formants from fine-structure to F0-rate fluctuations in the healthy ear; worth verifying how this maps to modulation transfer functions measured behaviorally in impaired listeners.